Flavins bind to egg riboflavin proteins with all thermodynamic parameters favoring the interaction, the state of ionization of the phosphate is unimportant, FAD binding at low pH causes the concomitant release of about 1 mol of protons per mol of complex formed, and no significant conformations/change seems to occur concomitant with binding. These conclusions are based on a thorough thermodynamic study of the binding reaction and are used to develop a conceptual model of the binding process and the protein binding site. It is proposed to extend these studies to other flavin binding systems, specifically those from Azobacter vinelandii and Clostridium pastureanum.